This invention is an improved terpolymer obtained by copolymerizing vinylidene fluoride, hexafluoropropene and tetrafluoroethylene. The resulting fluoroelastomer possesses both improved low temperature compression set and a low glass transition temperature.
Fluoroelastomeric terpolymers are known in the art. U.S. Pat. No. 2,968,649 discloses and claims a terpolymer consisting of 3 to 35% by weight of tetrafluoroethylene units and 97 to 65% by weight of vinylidene fluoride and hexafluoropropene units, in which the vinylidene fluoride and hexafluoropropene units are in a weight ratio in the range of 2.33:1 to 0.667:1.0
U.S. Pat. No. 3,801.552 discloses a method of producing a fluoroelastomeric terpolymer by polymerization of tetrafluoroethylene, vinylidene fluoride and hexafluoropropene. The composition of the terpolymer falls within the area bounded by the following four mole ratios of tetrafluoroethylene, vinylidene fluoride, and hexafluorpropene: 5:85.7:9.3, 5:59.7:35.3, 30:28:42, and 30:54:16.
Japanese Patent Publication 48-18957/73 discloses a process for preparing fluorine-containing terpolymers containing 10-30% by weight of tetrafluoroethylene and 90-70% by weight of vinylidene fluoride and hexafluoropropene having weight ratio of vinylidene fluoride to hexafluoropropene in the range 1.6:1 to 4:1.
In addition, a fluorine-containing terpolymer has been marketed in the U.S. by Daikin Kogyo Co., Ltd., Osaka, Japan, in which the composition by weight of tetrafluoroethylene, vinylidene fluoride and hexafluoropropene was about 14:53:33.
Unless otherwise indicated, all percentages used herein denote weight percent, and vinylidene fluoride, tetrafluoroethylene, and hexafluoropropene will be referred to as VF.sub.2, TFE and HFP, respectively.